Broad-spectrum describes a chemical or medicine that works against a wide range of pests, germs, or diseases, not just one kind. Doctors talk about a broad-spectrum antibiotic when they need a drug that can hit many types of bacteria at once. Farmers and gardeners use the same idea for pesticides. Though actually, ‘broad’ is not always better. A medicine that kills many kinds of bacteria can also kill useful ones, so good doctors try not to reach for a broad-spectrum drug when a narrower one would do. The word is almost always used before a noun: a broad-spectrum antibiotic, a broad-spectrum pesticide. People have stretched it a little, too — for example, broad-spectrum sunscreen means a cream that covers a wide range of ultraviolet light — but the core picture is still ‘one thing that covers a wide range of targets.’